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Angular Correlation (angular + correlation)
Selected AbstractsThe Application of 199Hg NMR and 199mHg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of HgII: A Case Study with Designed Two- and Three-Stranded Coiled CoilsCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2007Olga Iranzo Dr. Abstract The use of de novo designed peptides is a powerful strategy to elucidate HgII,protein interactions and to gain insight into the chemistry of HgII in biological systems. Cysteine derivatives of the designed ,-helical peptides of the TRI family [Ac-G-(LaKbAcLdEeEfKg)4 -G-NH2] bind HgII at high pH values and at peptide/HgII ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting HgII complexes are good water-soluble models for HgII binding to the protein MerR. We have carried out a parallel study using 199Hg NMR and 199mHg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI,L9C/HgII ratios. These studies prove for the first time the formation of [Hg{(TRI,L9C)2 -(TRI,L9CH)}], a dithiolate,HgII complex in the hydrophobic interior of the three-stranded coiled coil (TRI,L9C)3. 199Hg NMR and 199mHg PAC data demonstrate that this dithiolate,HgII complex is different from the dithiolate [Hg(TRI,L9C)2], and that the presence of third ,-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI,L9C)2]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate,HgII complex Hg(TRI,L9C)3, on a timescale that is slower than the NMR timescale (0.01,10,ms). The formation of the species [Hg{(TRI,L9C)2(TRI,L9CH)}] is the result of a compromise between the high affinity of HgII to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil. [source] Room-Temperature Degradation of t -Zr(Pr)O2 in an Aqueous Suspension Revealed by Perturbed Angular CorrelationsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2008María C. Caracoche This paper deals with the phase stability of an aqueous suspension of tetragonal Zr0.9Pr0.1O2 (20 wt%/vol%) at room temperature as a function of the aging time. The suspension is investigated in situ using the highly localized Perturbed Angular Correlations technique. The results indicate that an almost fully reversible degradation process toward monoclinic zirconia takes place through a first-order reaction of rate constant k=0.7 day,1. Two successive diffusion mechanisms are observed that are interpreted as OH, ions' migration in the grain surface and then, as proton defects' diffusion into the bulk. [source] Nitrided Amorphous Stainless Steel Coatings Deposited by Reactive Magnetron Sputtering from an Austenitic Stainless Steel TargetADVANCED ENGINEERING MATERIALS, Issue 1-2 2009Salvatore Cusenza Abstract Stainless steel films were reactively magnetron sputtered in argon/nitrogen gas flow onto oxidized silicon wafers using austenitic AISI 316 stainless-steel targets. The deposited films of about 300,nm thickness were characterized by conversion electron Mö-i;ssbauer spectroscopy, magneto-optical Kerr-effect, X-ray diffraction, Rutherford backscattering spectrometry, and resonant nuclear reaction analysis. These complementary methods were used for a detailed examination of the nitriding effects for the sputtered stainless-steel films. The formation of an amorphous and soft ferromagnetic phase in a wide range of the processing parameters was found. Further, the influence of postvacuum-annealing was examined by perturbed angular correlation to achieve a comprehensive understanding of the nitriding process and phase formation. The amorphous phase is not very stable and crystallization can be observed at 973,K. [source] Estimation of the absolute internal-rotation entropy of molecules with two torsional degrees of freedom from stochastic simulationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2005Eva Darian Abstract A method of statistical estimation is applied to the problem of evaluating the absolute entropy of internal rotation in a molecule with two torsional degrees of freedom. The configurational part of the entropy is obtained as that of the joint probability density of an arbitrary form represented by a two-dimensional Fourier series, the coefficients of which are statistically estimated using a sample of the torsional angles of the molecule obtained by a stochastic simulation. The internal rotors in the molecule are assumed to be attached to a common frame, and their reduced moments of inertia are initially calculated as functions of the two torsional angles, but averaged over all the remaining internal degrees of freedom using the stochastic-simulation sample of the atomic configurations of the molecule. The torsional-angle dependence of the reduced moments of inertia can be also averaged out, and the absolute internal-rotation entropy of the molecule is obtained in a good approximation as the sum of the configurational entropy and a kinetic contribution fully determined by the averaged reduced moments of inertia. The method is illustrated using Monte Carlo simulations of isomers of stilbene and halogenated derivatives of propane. The two torsional angles in cis -stilbene are found to be much more strongly correlated than those in trans -stilbene, while the degree of the angular correlation in propane increases strongly on substitution of hydrogen atoms with chlorine. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 651,660, 2005 [source] Vacancy-type defects and electronic structure of perovskite-oxide SrTiO3 from positron annihilationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2006A. S. Hamid Abstract The vacancy-type defects in Nb-doped SrTiO3 and in undoped SrTiO3, annealed in H2 flow, were investigated by means of positron lifetime and 2D angular correlation of annihilation radiation (ACAR) experiments. The calculations of the lifetime of positron were performed by using atomic superposition (AT-SUP) method. The results showed that positrons annihilate from a free state in the Nb-doped SrTiO3. The trapping centers in the annealed sample were found to be oxygen vacancies VO associated with relaxation of the surrounding ions. Moreover, the momentum distributions of the samples studied were correlated to the variation of their electronic structure. It was proposed from the drastic change in the momentum distribution upon introduction of VO, that 2D-ACAR technique is a sensitive tool for acquiring information on the electronic and bond structure of the perovskite-oxides. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spin dependent distribution and Fermi surface of the perovskite manganite compound La0.7Sr0.3MnO3 via 2D-ACAR measurements,PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2004A. S. Hamid Abstract Using 2D angular correlation of positron annihilation radiation (ACAR) experiment, we have performed a systematic study of the spin dependent and Fermi surface of the colossal magneto-resistance CMR La0.7Sr0.3MnO3. The measurements have been carried out using a re-versal magnetic field direction (parallel and anti-parallel to the direction of motion of the polarized posi-trons). The measured spectra have been investigated in the momentum space as well as in the wave vector space. They revealed information about the hybridization effect of Mn(3d eg1) and O(2p) like states. Further, the results showed that the majority spin electrons predominated at the Fermi level. From another perspective, the Fermi surface of La0.7Sr0.3MnO3 revealed a cuboids hole surface centered on R point and a spheroid electron surface centered on , point. A comparison with the earlier results showed qualitative agreement. However, the current results could reveal the dimension of the electron surface centered on , point that was predicted in the earlier 2D-ACAR measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Many body effects seen in the positron annihilation experimentPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007G. Kontrym-Sznajd Abstract The electron-positron (e-p) momentum density in p -space for copper, magnesium, cadmium and yttrium, calculated by using various theoretical approaches, are compared with corresponding densities reconstructed from two-dimensional angular correlation of annihilation radiation (2D ACAR) experimental spectra. We demonstrate that (at least for positrons in metals) strong electron-electron (e-e) correlations are clearly visible not only in the Compton scattering but also in positron annihilation experiments. Moreover, a proper description of e-p correlations needs to include the lattice-periodical crystal potential. In case of strong lattice effects, the increase of the e-p enhancement with increasing momentum - the so-called Kahana-like enhancement, is vanishing. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface and interface investigations using radioactive atomsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004K. Potzger Abstract The magnetic behavior of isolated nonmagnetic probe atoms (Cd) at ferromagnetic Ni surfaces and interfaces have been studied using perturbed angular correlation (PAC) spectroscopy. Atomic resolution is achieved making it possible to distinguish between probe atoms at various surface sites, i.e., in/at terraces, atomic steps, kinks. These different atomic surface sites have differing Ni coordination numbers, (i.e., the number of Ni nearest neighbours). With decreasing coordination number, a continuous nonlinear dependent correlation was found with increasing magnetic hyperfine fields (Bhf). Furthermore, experimental evidence is presented, that the induced s-moments on the Cd isolated impurity is independent of the details of the surface symmetry but essentially determined only by the coordination number. Finally, it has been observed that monolayers of Pd on Ni form large unit cells, which offer selected sites to the Cd impurity. We discuss this finding as basic research towards future applications with self-organizing patterned structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Application of 199Hg NMR and 199mHg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of HgII: A Case Study with Designed Two- and Three-Stranded Coiled CoilsCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2007Olga Iranzo Dr. Abstract The use of de novo designed peptides is a powerful strategy to elucidate HgII,protein interactions and to gain insight into the chemistry of HgII in biological systems. Cysteine derivatives of the designed ,-helical peptides of the TRI family [Ac-G-(LaKbAcLdEeEfKg)4 -G-NH2] bind HgII at high pH values and at peptide/HgII ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting HgII complexes are good water-soluble models for HgII binding to the protein MerR. We have carried out a parallel study using 199Hg NMR and 199mHg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI,L9C/HgII ratios. These studies prove for the first time the formation of [Hg{(TRI,L9C)2 -(TRI,L9CH)}], a dithiolate,HgII complex in the hydrophobic interior of the three-stranded coiled coil (TRI,L9C)3. 199Hg NMR and 199mHg PAC data demonstrate that this dithiolate,HgII complex is different from the dithiolate [Hg(TRI,L9C)2], and that the presence of third ,-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI,L9C)2]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate,HgII complex Hg(TRI,L9C)3, on a timescale that is slower than the NMR timescale (0.01,10,ms). The formation of the species [Hg{(TRI,L9C)2(TRI,L9CH)}] is the result of a compromise between the high affinity of HgII to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil. [source] | ||||||||||||||||